1. Field of the Invention
The present invention relates to an image heating apparatus which can be used as a heating and fixing apparatus (fixing device) that is mounted on an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer, and to a heater used for the image heating apparatus.
2. Description of the Related Art
Some heating and fixing apparatuses (fixing device) which are mounted on an electrophotographic printer or an electrophotographic copying machine have a heater having a heat generating resistor on a substrate made from ceramic, a flexible member (fixing film) which moves while contacting the heater, and a pressure roller which forms a nipping portion with the heater through the flexible member. A recording medium which carries an unfixed toner image thereon is heated while being sandwiched in the nipping portion of the fixing device and transported therethrough, and thereby, an image on the recording medium is heated and fixed on the recording medium. This fixing device has the advantage of spending a short period of time for raising the temperature of the heater to a fixable temperature after having started the energization of the heater. Accordingly, a printer having this fixing device mounted thereon can shorten the period of time (FPOT: First Print Out Time) for outputting the first image after a print command has been input. This type of a fixing device has also the advantage of consuming little electric power in a period in which it waits for the print command.
By the way, it is known that when a recording medium with a small size is continuously printed at the same printing interval as that for a recording medium with a large size by using a printer that mounts a fixing device thereon which uses the flexible member, the temperature of a region of the heater through which the recording medium does not pass (non-feeding region) excessively increases. When the non-feeding region of the heater excessively increases in temperature, the heat occasionally damages the holder that holds the heater and the pressure roller.
Therefore, when the printer that mounts the fixing device thereon, which uses the flexible member, continuously prints an image on a recording medium with a small size, the printer controls itself so as to extend a printing interval wider than in the case of continuously printing a recording medium with a large size, and inhibits an excessive rise in the temperature of the non-feeding region of the heater.
However, the control for extending the printing interval reduces the number of sheets to be output per unit time, and the number of sheets to be output per unit time is desired to be controlled so as to be equivalent to or slightly less than that in the case of printing the recording medium with the large size.
For this reason, it is considered to use a material having such negative resistance-temperature characteristics (NTC: Negative Temperature Coefficient) that the resistance value decreases as the temperature rises, for the heater used in the above-described fixing device. This is a concept that when the heater has negative resistance-temperature characteristics, the resistance value in the non-feeding region decreases even though the temperature of the non-feeding region has excessively increased, and accordingly can inhibit an excessive rise in the temperature of the non-feeding region.
However, a heat generating resistor having negative resistance-temperature characteristics generally has high volume resistance, and it is often difficult to obtain electric resistance in a range in which a commercial power source is usable, from a normal heat generating resistor pattern.
Japanese Patent Application Laid-Open No. 2007-025474 proposes a heating member which is manufactured so as to obtain a resistance in a range in which a commercial power source is useful even when using the heat generating resistor having the negative resistance-temperature characteristics. This heating member has heat generating resistors having negative resistance-temperature characteristics such as graphite, for instance, divided in a longitudinal direction of a substrate; supplies electric power to one area of the divided heat generating resistors in a transverse direction of the substrate (transport direction of the recording medium); and connects the divided heat generating resistor areas to each other in series. By employing a heating member having a heat generating resistor pattern having such a configuration, the temperature rise in the non-sheet feeding portion could be lowered with a simple configuration.
The above-described conventional heating member is desired to prevent the temperature in the non-sheet feeding portion from rising and simultaneously secure fixing properties in a gap between the divided heat generating resistors.